WASHINGTON - THE United States on Tuesday warned Iran it risked further isolation and new international sanctions after refusing to comply with UN Security Council resolutions over its disputed nuclear programme.

US Secretary of State Condoleezza Rice was sceptical about Iran's claim that it had started work to install 6,000 new centrifuges to enrich uranium at its main nuclear plan in Natanz.

'I can't substantiate the claims. There are always multiple claims coming out of Iran about progress on this, progress on that. I don't think the underlying situation has changed,' Dr Rice told reporters.

'The Iranian government continues to be in violation of multiple United Nations Security Council resolutions and with each step it takes it continues to isolate its people and risk further international financial and diplomatic sanctions,' he said.

The announcement came as Iran marked its 'national day of nuclear technology' on the second anniversary of its first production of uranium sufficiently enriched to make atomic fuel.

Iran has already installed around 3,000 P1 centrifuges at an underground enrichment facility at Natanz, in central Iran, according to the latest report by the UN nuclear watchdog. Tripling that number would mark a major expansion of its nuclear capacities.

The West fears Iran could use enriched uranium to make a nuclear weapon, and Teheran's refusal to suspend the process has been punished with three sets of UN Security Council sanctions resolutions and US pressure on its banking system.

Mr McCormack said that, although Iran was benefiting from record high oil prices, the sanctions 'exacerbate the effects' of the Iranian government's 'mismanagement' of the economy.

'You have increasing rates of inflation there and the need for them to dip into some of their reserves in order to fund their budget and to fund the extravagant promises that Ahmadinejad has made to the population,' he said.

The US government is hoping the sanctions will put increasing popular pressure on the Iranian leadership to change course.

Mr McCormack said neither Washington nor the international community wanted 'to cause any harm or hardship to the Iranian people, but they have a government that was making these choices on their behalf.'

Though he did not see all of Mr Ahmadinejad's charge that Washington used the Sept 11th attacks as a pretext to attack Afghanistan and Iraq, Mr McCormack said: 'We went into Afghanistan because that's where the attack originated.' As for invading Iraq, he replied: 'I think we have talked about those at length and everybody understands very clearly what our reasons for doing that were.'